Field of the Invention
The present invention is related to a system of contact lenses, each lens having a different level or degree of stabilization corresponding to a vision correction for a non-rotationally symmetric eye aberration and to a method of optimizing comfort for a patient by varying stabilization design as a function of a required vision correction.
Discussion of Related Art
Myopia or nearsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point before they reach the retina. Myopia generally occurs because the eyeball or globe is too long or the cornea is too steep. A minus or negative powered spherical lens may be utilized to correct myopia. Hyperopia or farsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point after they reach or behind the retina. Hyperopia generally occurs because the eyeball or globe is too short or the cornea is too flat. A plus or positive powered spherical lens may be utilized to correct hyperopia.
Astigmatism is an optical or refractive defect in which an individual's vision is blurred due to the inability of the eye to focus a point object into a focused image on the retina. Astigmatism is caused by a non-rotationally symmetric curvature of the refracting surfaces of the eye (including the cornea and crystalline lens). A non-astigmatic eye has rotationally symmetric refracting surfaces whereas in an individual with astigmatism, the refracting surfaces are not rotationally symmetrical. In other words, one or more of the refracting surfaces are more curved or steeper in one principal meridian relative to the other orthogonal principal meridian, thereby causing an image to be stretched out into two line foci rather than focused to a single point. A non-rotationally symmetric lens rather than a rotationally symmetric lens may be utilized to resolve astigmatism.
Contact lenses may be utilized to correct myopia, hyperopia, astigmatism as well as other visual acuity defects. Contact lenses may also be utilized to enhance the natural appearance of a wearer's eyes. Contact lenses or contacts are simply lenses placed on the anterior surface of the eye. Contact lenses are considered medical devices and may be worn to correct vision and/or for cosmetic or other therapeutic reasons. Early contact lenses were made or fabricated from hard materials, were relatively expensive, and were fragile. In addition, these early contact lenses were fabricated from materials that did not allow sufficient oxygen transmission through the contact lens to the conjunctiva and cornea, which potentially could cause a number of adverse clinical effects. Although these contact lenses are still utilized, they are not suitable for all patients due to their poor initial comfort. Later developments in the field gave rise to soft contact lenses, based upon hydrogels, which are extremely popular and widely utilized today. Specifically, silicone hydrogel contact lenses combine the benefit of silicone, which has extremely high oxygen permeability, with the proven comfort and clinical performance of hydrogels. Essentially, these silicone hydrogel based contact lenses have higher oxygen permeabilities and are generally more comfortable to wear than the contact lenses made of the earlier hard materials.
Contact lenses may be broadly categorized as daily disposable contact lenses, frequent replacement contact lenses and traditional contact lenses. Daily disposable contact lenses, as the name implies, are worn for a single day and discarded. Cleaning solutions are not typically utilized with these lenses. Frequent replacement contact lenses include lenses that may be reused for two weeks up to one month depending on the manufacturer's and/or the doctor's recommendation and generally require cleaning and disinfection every day. There are even contact lenses that are approved for longer periods of reuse. Frequent replacement lenses also include extended wear contact lenses which may be worn during sleep. Traditional contact lenses or reusable contact lenses are worn for much longer periods and are typically discarded approximately every six months.
Corneal astigmatism may be corrected using a hard or rigid gas permeable contact lens. In this case, a fluid or tear lens may exist between the posterior surface of the rigid contact lens and the cornea. This fluid or tear lens follows or assumes the shape of the back surface of the contact lens. Since the index of refraction of the fluid or tear lens is nearly a match for the cornea, the corneal toricity is optically neutralized or reduced. In these cases, a toric lens will generally not be required. However, rigid gas permeable contact lenses and hard contact lenses are generally less comfortable than soft or hydrogel contact lenses. Since soft or hydrogel contact lenses wrap around the cornea, a fluid lens is generally not found and the tear fluid more closely resembles a thin film. In this case, a toric lens design is required.
A toric lens is an optical element having two different powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical, for correcting myopia or hyperopia and one power, cylinder, for correcting astigmatism built into a single lens. These powers are created with curvatures oriented at different angles which are preferably maintained relative to the eye. Toric lenses may be utilized in eyeglasses, intraocular lenses, and contact lenses. The toric lenses used in eyeglasses and intraocular lenses are held fixed relative to the eye by either the spectacle frame or haptics thereby always providing optimal vision correction. However, toric contact lenses may tend to rotate on the eye, thereby temporarily providing sub-optimal vision correction. Accordingly, currently-utilized toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around. For many high order aberrations, many of which are not rotationally symmetric, positional stability is also required to provide optimal vision correction.
The use of contact lenses is problematic in that each contact lens of the pair must be maintained at a specific orientation while on the eye to be effective. When the contact lens is first placed on-eye, it must automatically position, or auto-position, itself and then maintain that position over time. However, once the contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking as well as eyelid and tear film movement.
Maintenance of the on-eye orientation of a contact lens typically is accomplished by altering the mechanical characteristics of the contact lens. For example, prism stabilization, including decentering or tilting of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge, are all methods that have been utilized.
Additionally, static stabilization has been used in which the contact lens is stabilized by the use of thick and thin zones, or areas in which the thickness of the contact lens' periphery is increased or reduced, as the case may be. Typically, the thick and thin zones are located in the contact lens' periphery with symmetry about the vertical and/or horizontal axes. For example, each of two thick zones may be positioned on either side of the optic zone and centered along the 0-180 degree axis of the contact lens. In another example, a single thick zone positioned at the bottom of the contact lens providing a similar weight effect, like that of prism stabilization, but also incorporating a region of increasing thickness from top to bottom in order to utilize upper eyelid forces to stabilize the contact lens may be designed. It is important to note that the older technical literature utilizes the term dynamic stabilization for what is meant here as static stabilization. Accordingly, for purposes of this invention static and dynamic stabilization may be utilized interchangeably.
The challenge with currently designed or utilized stabilization zones is a tradeoff between contact lens stability and comfort, plus the physical limitations associated with increased thickness. With a static or dynamic stabilization zone, the slope of the stabilization zone is fixed in the contact lens. Changes to the design to improve rotational speed, such as increasing the surface slope of the stabilization zone, also increases contact lens thickness and may adversely impact comfort. Additionally, the contact lens design has to accomplish two things; namely, to rotate to the proper orientation on insertion, and to maintain that orientation through the wear period. Conventional designs require tradeoffs in performance between these two modes.
The higher the amount of cylinder correction designed into a contact lens, the more sensitive the lens is to axis misalignments and rotational stability on eye in terms of adversely affecting the wearer's visual acuity. Therefore, a higher cylinder correction requires a robust stabilization mechanism in the lens design. Such a stabilization mechanism, however, can cause an increased awareness for patients, as the eyelids interact with the mechanical stability features on the lens. For lower cylinder corrections, lens designs are less sensitive to axis misalignments and rotational stability, thereby allowing more rotation from the nominal position for similar impact on optical quality compared to higher cylinder corrections.